Improving the quantitative estimation of trawling impacts from sidescan-sonar and underwater-video imagery

Abstract

The techniques of sidescan sonar and towed, underwater-video sled were assessed as rapid-assessment methodologies for investigating trawl impacts on the substratum. Sidescan sonar is able to image a swathe of ∼200 m with a resolution of ∼20 cm at a speed of 2-3 knots, and marks of trawl doors could be observed. The towed video system imaged a swathe of 1-2 m with a resolution of 1-2 cm at a speed of ∼1 knot, and trawl-door marks, scrape marks, local bioturbation features, and fauna could be observed. Multiple tows using both methodologies were carried out in two areas in Heraklion Bay, Crete. One area, experimentally trawled, was 80-90 m deep and characterized by mixed, maerly sediments; the other was a commercial trawl lane ∼200 m deep characterized by silty-clay sediment. Descriptions of the types of trawling feature and impacts caused by trawling were made for both areas. Images were analysed from the commercial deeper trawling ground for area assessment. For sidescan-sonar records, direction of trawling and trawl-mark density by category were estimated at periodic intervals along the track. For video, categories for trawl-mark density and level of bioturbation were estimated, along with the density of the crinoid Leptometra phalangium. Using geo-referenced positioning for each data point, area maps were constructed for each of the parameters, and correlations were tested between the different datasets. The use of the assessment techniques (characteristics, data usage, mapping, complementarity) in relation to trawling-impact studies is discussed, as well as possibilities for the use of the resulting data for management. © 2007 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved.